This invention relates to the thermal barrier coating used to protect an article such as a nickel-base superalloy substrate and, more particularly, to the inhibiting of the sintering between the grains of the thermal barrier coating.
A thermal barrier coating system may be used to protect the components of a gas turbine engine that are subjected to the highest temperatures. The thermal barrier coating system usually includes a bond coat that is deposited upon a superalloy substrate, and a ceramic thermal barrier coating that is deposited upon the bond coat. The thermal barrier coating acts as a thermal insulator against the heat of the hot combustion gas. The bond coat bonds the thermal barrier coating to the substrate and also inhibits oxidation and corrosion of the substrate.
The currently preferred thermal barrier coating is yttria-stabilized zirconia (YSZ), which is zirconia (zirconium oxide) with from about 2 to about 12 percent by weight yttria (yttrium oxide) added to stabilize the zirconia against phase changes that otherwise occur as the thermal barrier coating is heated and cooled during fabrication and service. The YSZ is deposited by a physical vapor deposition process such as electron beam physical vapor deposition. In this deposition process, the grains of the YSZ form as columns extending generally outwardly from and perpendicular to the surfaces of the substrate and the bond coat.
When the YSZ is initially deposited, there are small gaps between the generally columnar grains. On examination at high magnification, the generally columnar grains are seen to have a somewhat feather-like morphology characterized by these gaps oriented over a variety of angles relative to the substrate surface. The gaps serve to accommodate the transverse thermal expansion strains of the columnar grains and also act as an air insulator in the insulator structure. As the YSZ is exposed to elevated temperatures during service, these gaps gradually close by a sintering mechanism. As a result, the ability of the YSZ to accommodate thermal expansion strains gradually is reduced, and the thermal conductivity of the YSZ gradually increases by about 20 percent or more. The as-deposited thickness of the YSZ must therefore be greater than would otherwise be desired, to account for the loss of insulating power associated with this rise in thermal conductivity during service.
It has been recognized that the addition of sintering inhibitors to the YSZ reduces the tendency of the gaps between the columnar grains to close by sintering during service of the thermal barrier coating. A number of sintering inhibitors have been proposed. However, these sintering inhibitors have various shortcomings, and there is a need for more effective sintering inhibitors. The present invention fulfills this need, and further provides related advantages.
The present invention provides an article protected by a thermal barrier coating system, and a method for its fabrication. The thermal barrier coating includes effective sintering inhibitors that slow or prevent the closure of the gaps between the columnar grains. The sintering inhibitors are readily introduced into the thermal barrier coating by an infiltration technique or by co-evaporation, preferably from an ingot that contains the sintering inhibitors.
An article protected by a thermal barrier coating system comprises a substrate having a substrate surface, and a thermal barrier coating system overlying the substrate. The thermal barrier coating system comprises a thermal barrier coating formed of a thermal barrier coating material arranged as a plurality of columnar grains extending generally perpendicular to the substrate surface and having grain surfaces, and a sintering inhibitor within the columnar grains. The sintering inhibitor may be uniformly distributed within the columnar grains or concentrated at the grain surfaces. The sintering inhibitor is selected from the group consisting of lanthanum oxide, lanthanum chromate, chromium oxide, and yttrium chromate, mixtures thereof, mixtures thereof with aluminum oxide, modifications thereof wherein cobalt or manganese is substituted for chromium, precursors thereof, and reaction products thereof. These are the xe2x80x9cformsxe2x80x9d of the sintering inhibitor. Preferably, a bond coat is disposed between the substrate and the thermal barrier coating. The bond coat may be a diffusion aluminide or an aluminum-containing overlay coating, and is most preferably a platinum aluminide.
The substrate is preferably a nickel-base superalloy in the form of a component of a gas turbine engine. Examples include a turbine blade, a turbine vane, and combustor components such as fuel nozzles and shields.
A method for fabricating an article protected by a thermal barrier coating system comprises the steps of providing a substrate having a substrate surface, and applying a thermal barrier coating system overlying the substrate. The thermal barrier coating system comprises a thermal barrier coating formed of a thermal barrier coating material arranged as a plurality of columnar grains extending generally perpendicular to the substrate surface and having grain surfaces, and a sintering inhibitor within the columnar grains. The sintering inhibitor is selected from the group consisting of lanthanum oxide, lanthanum chromate, chromium oxide, and yttrium chromate, mixtures thereof, mixtures thereof with aluminum oxide, modifications thereof wherein cobalt or manganese is substituted for chromium, precursors thereof, and reaction products thereof. These are the xe2x80x9cformsxe2x80x9d of the sintering inhibitor.
In one embodiment, the thermal barrier coating is deposited, and thereafter the sintering inhibitor is infiltrated into the thermal barrier coating. In another embodiment, the thermal barrier coating material and the sintering inhibitor are co-deposited. In the various embodiments, a bond coat is preferably deposited on the substrate surface prior to depositing the thermal barrier coating.
The sintering inhibitor provides effective inhibition of the sintering that otherwise closes the gaps between the columnar ceramic grains of the thermal barrier coating during service. The result is that the ability of the thermal barrier coating to withstand the development of thermally induced stresses is retained for an extended service lifetime. The thermal conductivity of the thermal barrier coating is also maintained at a low level for an extended period. Some of the species may additionally improve the optical and thermal properties of the thermal barrier coating. Chromium oxide, for example, alters the emissivity of the thermal barrier coating in the 0.7-1 micrometer wavelength range.
Other features and advantages of the present invention will be apparent from the following more detailed description of the preferred embodiment, taken in conjunction with the accompanying drawings, which illustrate, by way of example, the principles of the invention. The scope of the invention is not, however, limited to this preferred embodiment.